Fix custom marshaler for enum types

[olegshaldybin]

When MarshalJSON was defined on a pointer receiver custom enum type
marshaling/unmarshaling was panicing since the underlying primitive type
was treated as a pointer.

Since method set for pointer receivers includes value receiver methods
we don't really need optionalEncoder and can just use marshalEncoder
directly.

--- FAIL: Test_custom_marshaler_on_enum (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x1 pc=0x133d7a6]

goroutine 6 [running]:
testing.tRunner.func1(0xc42007b1e0)
	/usr/local/go/src/testing/testing.go:622 +0x29d
panic(0x13eb880, 0x1647d90)
	/usr/local/go/src/runtime/panic.go:489 +0x2cf
github.com/olegshaldybin/go.(*MyEnum).MarshalJSON(0x1, 0x13ec960, 0x1, 0x179b0e0, 0x1, 0xc4200c39e0)
	/Users/oleg/go/src/github.com/olegshaldybin/go/jsoniter_enum_marshaler_test.go:18 +0x26
github.com/olegshaldybin/go.(*marshalerEncoder).Encode(0xc4200f1300, 0x1, 0xc420018b40)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect_native.go:665 +0x6b
github.com/olegshaldybin/go.(*optionalEncoder).Encode(0xc42010e6a0, 0xc42010e5d0, 0xc420018b40)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect.go:124 +0x4f
github.com/olegshaldybin/go.WriteToStream(0x13c81e0, 0xc42010e5d0, 0xc420018b40, 0x1622ce0, 0xc42010e6a0)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect.go:49 +0x10b
github.com/olegshaldybin/go.(*optionalEncoder).EncodeInterface(0xc42010e6a0, 0x13c81e0, 0xc42010e5d0, 0xc420018b40)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect.go:129 +0x55
github.com/olegshaldybin/go.(*Stream).WriteVal(0xc420018b40, 0x13c81e0, 0xc42010e5d0)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect.go:234 +0xe4
github.com/olegshaldybin/go.(*emptyInterfaceCodec).Encode(0x1670410, 0xc42010e5e0, 0xc420018b40)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect_native.go:368 +0x41
github.com/olegshaldybin/go.(*structFieldEncoder).Encode(0xc4200f1240, 0xc42010e5e0, 0xc420018b40)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect_object.go:118 +0x5c
github.com/olegshaldybin/go.(*structEncoder).Encode(0xc4200f12c0, 0xc42010e5e0, 0xc420018b40)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect_object.go:155 +0xc2
github.com/olegshaldybin/go.(*structEncoder).EncodeInterface(0xc4200f12c0, 0x13f8e80, 0xc42010e5e0, 0xc420018b40)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect_object.go:175 +0x145
github.com/olegshaldybin/go.(*Stream).WriteVal(0xc420018b40, 0x13f8e80, 0xc42010e5e0)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_reflect.go:234 +0xe4
github.com/olegshaldybin/go.(*frozenConfig).Marshal(0xc4200bcc40, 0x13f8e80, 0xc42010e5e0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_config.go:257 +0xdb
github.com/olegshaldybin/go.Marshal(0x13f8e80, 0xc42010e5e0, 0x13f8e80, 0xc42010e5e0, 0x1, 0x3, 0xc420102340)
	/Users/oleg/go/src/github.com/olegshaldybin/go/feature_adapter.go:43 +0x49
github.com/olegshaldybin/go.Test_custom_marshaler_on_enum(0xc42007b1e0)
	/Users/oleg/go/src/github.com/olegshaldybin/go/jsoniter_enum_marshaler_test.go:42 +0xd8
testing.tRunner(0xc42007b1e0, 0x1471208)
	/usr/local/go/src/testing/testing.go:657 +0x96
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:697 +0x2ca
exit status 2

[codecov]

Codecov Report

Merging #170 into master will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
- Coverage   84.78%   84.55%   -0.24%     
==========================================
  Files          41       41              
  Lines        5200     5199       -1     
==========================================
- Hits         4409     4396      -13     
- Misses        631      639       +8     
- Partials      160      164       +4

Impacted Files	Coverage Δ
feature_reflect_native.go	64.81% <ø> (ø)	⬆️
feature_reflect.go	77.53% <ø> (-0.05%)	⬇️
feature_iter_object.go	67.12% <0%> (-5.48%)	⬇️
feature_iter_skip.go	74.69% <0%> (-3.62%)	⬇️
feature_iter_string.go	88.81% <0%> (-0.7%)	⬇️

---

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